During acute fasting (12–24 hours), the liver maintains blood glucose primarily through glycogenolysis. Which allosteric effector activates hepatic glycogen phosphorylase b to phosphorylase a conversion by promoting its phosphorylation?

• A ATP, which binds the allosteric site and activates phosphorylase kinase
• B Glucose-6-phosphate, which signals adequate substrate and activates the enzyme
• C AMP, which activates phosphorylase b directly, while glucagon-driven cAMP/PKA activates phosphorylase kinase to convert b→a ✓
• D NADH, which allosterically activates phosphorylase kinase

Explanation

Glycogen phosphorylase exists as the less-active b form (dephosphorylated) and the active a form (phosphorylated at Ser-14). In the liver during fasting, glucagon raises cAMP → PKA → phosphorylase kinase activation → phosphorylase b phosphorylated to phosphorylase a. Simultaneously, AMP (which accumulates as energy charge falls) binds the allosteric site of phosphorylase b and partially activates it even without phosphorylation. ATP and glucose-6-phosphate are negative allosteric modulators of phosphorylase, signalling energy sufficiency. NADH is not a regulator of phosphorylase kinase.

Reference: Harper's Illustrated Biochemistry, 32nd ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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